1. Field of the Invention
The present invention concerns a method of inserting a message such as a secret mark in a digital signal.
It also concerns a method of extracting a message inserted in a digital signal.
2. Description of the Related Art
In a complementary manner, the present invention concerns a device for inserting a message and a device for extracting the message, adapted respectively to implement the insertion and extraction methods according to the invention.
The digital signal considered hereinafter will more particularly be a digital image signal.
The message insertion envisaged in the context of the invention lies within the technical field of the watermarking of digital data, which can be interpreted as the insertion of a seal in the digital data, making it possible for example to authenticate the content of a digital data file. This watermarking is also referred to as digital tattooing.
Watermarking entails in general terms the modification of coefficients representing the digital image. This modification is imperceptible to the eye, but can be decoded by an appropriate decoder.
The general aspects of watermarking are set out in particular by I. J. Cox, M. L. Miller and J. A. Bloom in “Digital watermarking”, Morgan Kaufmann Publishers, Academic Press, 2002.
The present application is concerned with the robust insertion of a message.
The insertion of a message is said to be robust if the message can later be extracted even if the image has undergone geometric distortions such as the cropping of a part of the image, a change of scale or rotation.
Solutions have been proposed for obtaining such robustness. A first category of solutions consists in preserving a reference of the position, of the size and of the orientation of the original image.
On extraction of the message, a prior phase of re-localizing is necessary, consisting of finding the reference again.
The document U.S. Pat. No. 5,636,292 proposes implementation of a method with re-localizing.
In this document, a signal preserving the reference information is inserted as the second watermarking signal. More particularly, a recognizable pattern is inserted in the magnitude information of the DFT (Discrete Fourier Tranform) transformed field. This field of representation of the image is invariable with respect to translations and cropping. The pattern serves later for re-localizing in case of change of scale or rotation of the image.
The inserted reference signal is a watermarking signal, and its use thus reduces the capacity of the image considered.
A second category of solutions consists of applying a watermarking method which is invariable with respect to geometrical transformations.
For example, the document by M. Alghoniemy and A. Tewfik entitled “Geometric distortion correction through image normalization” in Proc. ICME, New York, August 2000, proposes to normalize the image prior to the insertion of the message. Next, the same operation of normalization is carried out prior to the extraction of the message. The insertion and extraction are thus performed in the same normalized space, determined by invariable moments of the image.
These supplementary steps of normalization give rise to visual distortions in the image. Moreover, by that method, the insertion is not robust to cropping of the image.